1. Field of the Invention
The present invention generally relates the a field of disk drives, and more particularly to a head positioning control technique.
2. Description of the Related Art
Conventionally, in a disk drive as typified by a hard disk drive, a head positioning control system used to position a head (magnetic head) is provided at a target position (track as an access target) on a disk data storage medium. The head executes a read/write operation of data at the target position on the disk.
The head positioning control system includes a feedback control system using a microprocessor (CPU) as a controller. The feedback control system drives and controls a voice coil motor (VCM) of an actuator having the head mounted thereon and executes follow-up control over the head relative to the target position.
As the head positioning control system, there has been developed or proposed a system including a disturbance feedforward control system (Sometimes also called an “FF control system”) as well as the feedback control system. The FF control system suppresses the influence of vibrations or impacts (which will be generically referred to as disturbance hereinafter) applied from the outside to the disk drive.
The FF control system calculates a compensation value used to suppress the influence of the disturbance based on an acceleration value of the disturbance detected (observed) by an acceleration sensor provided in the disk drive. The head positioning control system uses the compensation value calculated by the FF control system, and absorbs a fluctuation generated by the disturbance when performing the follow-up control over the head in the feedback control system.
The FF control system includes a controller (which will be referred to as an FF controller hereinafter) having a predetermined transfer characteristic (determined as a transfer function F). The FF controller executes a digital filtering calculation using the acceleration value of the disturbance obtained from the acceleration sensor as an input. The head positioning control system adds the compensation value, which is an arithmetic operation result from the FF control system, to a control value calculated by the feedback control system, thereby suppressing a fluctuation in head position error generated due to the disturbance.
In short, in a head positioning control system including the FF control system, compensation relative to the disturbance applied during drive of the disk drive can be realized. In the head positioning control, therefore, a fluctuation in position error generated due to the disturbance can be suppressed, thereby improving the head positioning accuracy.
The performance of the FF control system depends on settings of the transfer function F of the FF controller. Usually, the transfer function F is determined based on a fluctuation characteristic of an acceleration of the disturbance (determined as a transfer function K), a position fluctuation characteristic obtained by the disturbance (determined as a transfer function W), a characteristic of a plant (determined as a transfer function P) and others. Here, the plant means an actuator including the VCM as a control target. The characteristic of the plant means, therefore, a mechanical characteristic (rigidity, resonance, damping factor) of the actuator.
Further, in actual disk drives, there are mechanical irregularities in each drive product, an attachment angle of an acceleration sensor, a relative angle of a head positioning location, a direction of the disturbance, an age-based change. Therefore, the transfer functions (K, W and P) in each of the above-described elements vary. Accordingly, it is desirable to adopt a method of determining the appropriate transfer function F of the FF control system by measuring these transfer functions when using the disk drive. If such a method is used, stable control against disturbances can be executed, and an improvement in the head positioning accuracy can be expected.
For these reasons, as a head positioning control system, a system including a parameter identification function used to determine the transfer function F of the FF control system is known. If such a system is employed, the head positioning accuracy with respect to the disturbance can be improved by updating the transfer function F in accordance with mechanical irregularities of each drive product or a change in environment during use.
Meanwhile, in general, as for the operation of the head positioning control system, control of the FF control system and the parameter identification processing of the system are different operations. Therefore, in cases where the disturbance such as sweep excitation occurs, if the head positioning accuracy is considerably deteriorated due to an age-based fluctuation in a disturbance value during execution of the control, the operation of the FF control system must be temporarily stopped. That is, after executing the parameter identification processing of the FF control system, the operation to again execute the control of the system is required. In such a method, the head positioning accuracy is temporarily deteriorated during execution of the parameter identification processing. Furthermore, since an algorithm to perform a switching operation from the control execution to the parameter identification operation is required, there is a problem that firmware (program of the CPU) is complicated.